Trim any wilted or brown spots from the chard leaves; trim the stem ends. Wash thoroughly in cold water and drain on kitchen towels. Using a shape paring knife, separate the thick stalks from the leaves. Cut the stalks into 1 inch pieces; finely shred the leaves. Keep separate. Peel the celery root and cut into 1 inch cubes. Keep in a bowl of cold water until ready to use.

In a 4 quart heavy pot or Dutch oven, heat the oil and butter over low heat; stir in the onion and chard stalks; sprinkle with salt; cover and sweat over a very low flame until the vegetables are very tender, about 20 minutes. (The vegetables should stew in a mixture of their own juices and the oil and butter.)

Drain the celery root and add to the pot along with the broth. Simmer, uncovered, until the celery root is tender, about 30 minutes. Puree the soup using a blender, immersion blender or foot mill fitted with the fine disk.

Return soup to the pot and simmer over low heat; stir in the chard leaves and cream. Season to taste with salt, pepper and lemon juice. Ladle into warm bowls, top with toasted croutons if desired and serve